CHANNELS OF THE LATER ST A (IE 467 , 



the last extending down to level of bottom land, which is about 10 feet 

 above the Missouri River. 



Three or four wells along the line of this valley from Missouri River 

 to Tabor show that the older rocks are about 100 feet below the surface, 

 which is at an altitude a little less than 1,350 feet. 



3. Another evidence of minor importance is the occurrence of boulders 

 of green quartzite of the Loup Fork group at the south end of a gravelly 

 ridge north of the junction of Beaver Creek and James River. The 

 source of these boulders probably is to the west, for green quartzite is 

 extensively exposed west of Xiobrara and, on the other hand, it is absent 

 to the north. 



THE ANCIENT PONCA 



The evidence of a former channel of Ponca Creek north of the Mis- 

 souri is very similar to that already given for the old Xiobrara. Its 

 course was from section 36, township 93 nortli, I'ange 62 west, northeast 

 past Perkins nearly to the valley of Emmanuel Creek, with which it 

 swings around south for some distance. It left that valley west of 

 Springfield station and finally entered the Xiobrara near where Em- 

 manuel Creek now joins the Missouri. 



1. The topographic evidence of this channel is similar to tliat of the 

 Xiobrara, especially in the fact that the general surface of the region 

 slopes toward it rather than toward the Missouri. The intervening- 

 divide is about 500 feet above the Missouri, while the ancient Ponca 

 channel is less than 200 feet above. 



2. There are old stream deposits, notably at a locality just east of a high 

 hill, on the east side of the junction of Choteau Creek and the Missouri. 

 Here there is quite a clear cross-section of the old stream deposits about 

 a half mile wide, with its bottom about 35 feet above the Missouri Rivei'. 

 The east bank of the old channel is well marked at this place, being 

 partly excavated in chalk : the west bank is obscured by the erosion of 

 recent streams. The channel is filled with 10 to 15 feet of gravel over- 

 lain with sand and clay and a ca])])ing of loain. which extends (o a height 

 140 to 150 feet above the river. 



A ravine cutting the side of the old channel deposit a few ]-ods form 

 the river has the following section : 



Feet 



Fine stratified sand and clay 24 



Coarse tjravel. with no northern houhlers 4 



Pea j?ravel and sand '2 



Chalk, bluish below 6 



The top of the chalk is about 30 feet above the river. On the opposite 

 side of the same creek the chalk rises about C^o feet above the river, while 



